As is well known, polyblends of rubber with styrene/acrylonitrile polymers have significant advantages in providing compositions of desirable resistance to impact for many applications. Various processes have been suggested or utilized for the manufacture of such polyblends including emulsion, suspension and mass polymerization techniques and combinations thereof. Although graft blends of a monoalkenyl aromatic and ethylenically unsaturated nitrile monomers and rubber prepared in mass exhibit desirable properties, this technique has a practical limitation upon the maximum degree of conversion of monomers to polymer which can be effected because of the high viscosities which are encountered when the reactions are carried beyond a fairly low degree of conversion after phase inversion takes place. As a result, techniques have been adopted wherein the initial polymerization is carried out in mass to a point of conversion beyond phase inversion at which the viscosity levels are still of practical magnitudes, after which the resulting prepolymerization syrup is suspended in water or other inert liquid and polymerization of the monomers carried to substantial completion.
N. E. Aubrey in U.S. Pat. No. 3,509,237 disclosed a mass/suspension method of polymerization styrene/acrylonitrile having diene rubbers dissolved therein with the rubber being grafted, inverted and dispersed as rubber particles under agitation. After phase inversion, the viscous mixture is suspended in water and polymerization is completed producing a polyblend in the form of beads.
Such mass/suspension processes are used commercially, however, present the economic problems of batch operations requiring long cycles at relatively low temperatures to control the heat of polymerization. Continuous mass polymerization processes have great economic advantages if they can be run at higher temperatures and higher rates with the necessary control of the great heats of polymerization. In the case of polyblends, the dispersed rubber phase must be formed and stabilized as to its morphology bringing it through the continuous polymerization of the rigid matrix polymer phase so that the physical properties of the polyblend meet exacting property specifications.
Various methods have been developed for the continuous mass polymerization of polyblends. Ruffing et.al. in U.S. Pat. No. 3,248,481 disclose a process wherein the diene rubbers are dissolved in predominantly monovinylidene aromatic monomers and polymerized in four reaction zones. Such processes require physically separated reactors providing different reacting conditions for each step of polymerization involving costly multiple reactors and specialized equipment.
Bronstert et.al. disclose in U.S. Pat. No. 3,658,946 a similar process wherein the prepolymerization step is run to a solids content of no more than 16% to provide a rubber particle having a particular structure. Bronstert et.al. disclose a need for separated nonstirred downstream reactors for final polymerization each providing a particular set of reacting conditions to insure final properties for the polyblend.
Okasaka et.al. disclose in U.S. Pat. No. 3,751,526 a process for producing rubber modified thermoplastic resins by extracting a grafted diene rubber from a latex into a styrene-acrylonitrile monomer phase, separating the monomer-rubber phase from the water phase and mass polymerizing the monomer-rubber phase to provide a polyblend of styrene/acrylonitrile polymer and a single grafted rubber phase.
N. E. Aubrey in U.S. Pat. No. 3,509,237 further discloses a process for preparing styrene/acrylonitrile/rubber polyblends having a first and second grafted rubber phase wherein the first grafted rubber has a large particle size and the second grafted rubber has a smaller particle size. Such polyblends have superior properties if the smaller particle size rubber phase constitutes the largest proportion of the total rubber phase. A process for making such polyblends is disclosed wherein the two grafted rubber polyblends are prepared in batch processes separately and thereafter melt blended mechanically to form a polyblend having a first and second grafted rubber phase.
Accordingly, it is an objective of the present invention to provide a continuous polymerization process that will produce rubber modified ABS type polymeric polyblends having a matrix polymer phase of predetermined average molecular weight and molecular weight distribution.
Another objective of this invention is to provide a continuous polymerization process for producing ABS type polyblends having first and second grafted rubber phase particles of predetermined morphology.
Another objective of this invention is to provide a continuous mass polymerization process with the necessary heat control using only one reaction zone minimizing the need for two or more reaction zones with extended conversion cycles and costly process equipment or the need for batch operations in preparing polyblends having two rubber phases having different morphology, in particular, a bimodal size distribution for the rubber particles.